The present invention relates to a reflective measuring scale graduation and to a method for its manufacture.
Incident light position-measuring instruments usually include a reflective measuring scale graduation, as well as a scanning device that is movable in relation thereto. A light source, arranged on the scanning-device radiates a beam of light in the direction of the reflective measuring scale graduation. The beam of light is then re-reflected in the direction of the scanning device where, modulated as a function of shift, the light passes through one or more scanning scale divisions to finally be detected by an optoelectronic detector system. The resulting shift-modulated sampled scanning signals are then further processed by a downstream evaluation unit.
Known reflective measuring scale graduations used in these position-measuring instruments usually include a substrate on which subdivisions having different reflection properties are arranged so as to alternate in the measuring direction. For example, a chromium coating may be applied over the entire surface of a steel substrate and then absorbent or low-reflecting subdivisions of chromium oxide (Cr2O3) may be arranged on top of the chromium layer. The highly reflecting subdivisions are then formed by the chromium subdivisions non-covered by the oxide.
Another known reflective measuring scale graduation is disclosed in U.S. Pat. No. 4,644,156. It describes using a photoresist to form non-reflective coated subdivisions on an aluminum substrate. The highly reflecting subdivisions are then formed by the exposed aluminum subdivisions.
Reflecting measuring scale graduations of this kind must meet a number of requirements. Such requirements include, inter alia, a high abrasion resistance, substantial reflectivity in the highly reflecting subdivisions, a high light absorption in the low reflecting subdivisions, and a low sensitivity to contamination. The variants of a reflective measuring scale graduation described above prove to be relatively sensitive to contamination. Thus, contamination from coolants or lubricants significantly reduces the reflectivity of the highly reflecting subdivisions, while substantially increasing the reflectivity in the low-reflecting subdivisions. Overall, the modulation factor of the scanning signals is clearly reduced by contamination when this measuring scale graduation is used in an optical reflected light position-measuring instrument.
To overcome this difficulty, German Patent 1,279,944 proposes forming the low-reflecting subdivisions of a reflective measuring scale graduation as anti-reflecting interference layers. The highly reflective subdivisions include gold layers. A drawback of this kind of reflective measuring scale graduation is that it requires an expensive manufacturing process. The requisite layer thickness of the dielectric interference layers must be precisely adjusted over the entire surface of the metal substrate. In addition, it is necessary to deposit a substantial number of individual partial layers on the substrate, making the manufacturing process even more costly.
The present invention is a reflective measuring scale graduation that is substantially insensitive to contamination and a method for manufacturing the same.
The reflective measuring scale graduation according to the present invention, as well as the method according to the present invention offer a number of advantages over previous approaches. For example, the sampled scanning signals maintain a high modulation factor even in the presence of contamination. There is also sufficient reflectivity in the highly reflecting subdivisions and adequate light absorption in the low-reflecting subdivisions of the measuring scale graduation in the presence of contamination. In addition, the reflecting measuring scale graduation according to the present invention also has a high mechanical load capacity. The method according to the present invention requires only a few process steps. For example, only one pattern delineation step is needed and no complex processes are required to adjust the requisite layer thickness.
In one aspect, the invention is a reflective measuring scale graduation comprising a reflective substrate, highly reflective first subdivisions disposed on the reflective substrate comprising a plurality of first partial layers and second partial layers, the plurality of first and second partial layers having different indices of refraction and forming an interference filter, and low reflecting second subdivisions comprising at least one absorber layer disposed on the reflecting substrate, the first and second subdivisions having different reflection properties and extending in at least one first direction on the reflecting substrate.
In a different aspect, the invention is a method for manufacturing a reflective measuring scale graduation comprising a first and second subdivisions having different reflection properties. The scale extends in at least one first direction on a reflecting substrate. The method comprises depositing a first partial layer having a low refraction index on the reflecting substrate, patterning the first partial layer so that the first partial layer covers the substrate in the first subdivisions, while the substrate is uncovered in the second subdivisions, and depositing of a second partial layer on the first and second subdivisions, the second partial layer having a greater refraction index than the first partial layer.
Further advantages as well as details pertaining to the present invention are included in the following description of the enclosed figures.